#include <functional>
 
#include <emscripten.h>
#include <SDL.h>
 
#define GL_GLEXT_PROTOTYPES 1
#include <SDL_opengles2.h>
 
// Shader sources
const GLchar* vertexSource =
"attribute vec4 position;                     \n"
"void main()                                  \n"
"{                                            \n"
"  gl_Position = vec4(position.xyz, 1.0);     \n"
"}                                            \n";
const GLchar* fragmentSource =
"precision mediump float;\n"
"void main()                                  \n"
"{                                            \n"
"  gl_FragColor[0] = gl_FragCoord.x/640.0;    \n"
"  gl_FragColor[1] = gl_FragCoord.y/480.0;    \n"
"  gl_FragColor[2] = 0.5;                     \n"
"}                                            \n";
 
// an example of something we will control from the javascript side
bool background_is_black = true;
 
// the function called by the javascript code
extern "C" void EMSCRIPTEN_KEEPALIVE toggle_background_color() { background_is_black = !background_is_black; }
 

SDL_Window * window;
GLfloat vertices[] = { 0.0f, 0.5f, 0.5f, -0.5f, -0.5f, -0.5f };

void main_loop() {
    const uint32_t milliseconds_since_start = SDL_GetTicks();
    const uint32_t milliseconds_per_loop = 3000;
    vertices[0] = (milliseconds_since_start % milliseconds_per_loop) / float(milliseconds_per_loop) - 0.5f;
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    // Clear the screen
    if (background_is_black)
        glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
    else
        glClearColor(0.9f, 0.9f, 0.9f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT);

    // Draw a triangle from the 3 vertices
    glDrawArrays(GL_TRIANGLES, 0, 3);

    SDL_GL_SwapWindow(window);
}
 
int main()
{
	SDL_CreateWindowAndRenderer(480, 640, 0, &window, nullptr);
 
	SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 2);
	SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 0);
	SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
	SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);
 
	// Create a Vertex Buffer Object and copy the vertex data to it
	GLuint vbo;
	glGenBuffers(1, &vbo);

 
	glBindBuffer(GL_ARRAY_BUFFER, vbo);
	glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
 
	// Create and compile the vertex shader
	GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);
	glShaderSource(vertexShader, 1, &vertexSource, nullptr);
	glCompileShader(vertexShader);
 
	// Create and compile the fragment shader
	GLuint fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
	glShaderSource(fragmentShader, 1, &fragmentSource, nullptr);
	glCompileShader(fragmentShader);
 
	// Link the vertex and fragment shader into a shader program
	GLuint shaderProgram = glCreateProgram();
	glAttachShader(shaderProgram, vertexShader);
	glAttachShader(shaderProgram, fragmentShader);
	glLinkProgram(shaderProgram);
	glUseProgram(shaderProgram);
 
	// Specify the layout of the vertex data
	GLint posAttrib = glGetAttribLocation(shaderProgram, "position");
	glEnableVertexAttribArray(posAttrib);
	glVertexAttribPointer(posAttrib, 2, GL_FLOAT, GL_FALSE, 0, 0);
 
	emscripten_set_main_loop(main_loop, 0, true);
 
	return EXIT_SUCCESS;
}